Will we find life — “not as we know it” — on Saturn’s moon Titan


Click to enlargeAzotosome, the theorized cell membrane on Titan

Liquid water is a requirement for life on Earth. But in other, much colder worlds, life might exist beyond the bounds of water-based chemistry.

Taking a simultaneously imaginative and rigidly scientific view, Cornell chemical engineers and astronomers offer a template for life that could thrive in a harsh, cold world – specifically Titan, the giant moon of Saturn. A planetary body awash with seas not of water, but of liquid methane, Titan could harbor methane-based, oxygen-free cells that metabolize, reproduce and do everything life on Earth does.

Their theorized cell membrane, composed of small organic nitrogen compounds and capable of functioning in liquid methane temperatures of 292 degrees below zero, is published in Science Advances…The work is led by chemical molecular dynamics expert Paulette Clancy…with first author James Stevenson, a graduate student in chemical engineering. The paper’s co-author is Jonathan Lunine…the College of Arts and Sciences’ Department of Astronomy…

On Earth, life is based on the phospholipid bilayer membrane, the strong, permeable, water-based vesicle that houses the organic matter of every cell. A vesicle made from such a membrane is called a liposome. Thus, many astronomers seek extraterrestrial life in what’s called the circumstellar habitable zone, the narrow band around the sun in which liquid water can exist. But what if cells weren’t based on water, but on methane, which has a much lower freezing point?

The engineers named their theorized cell membrane an “azotosome,” “azote” being the French word for nitrogen. “Liposome” comes from the Greek “lipos” and “soma” to mean “lipid body;” by analogy, “azotosome” means “nitrogen body.”

The azotosome is made from nitrogen, carbon and hydrogen molecules known to exist in the cryogenic seas of Titan, but shows the same stability and flexibility that Earth’s analogous liposome does. This came as a surprise to chemists like Clancy and Stevenson, who had never thought about the mechanics of cell stability before; they usually study semiconductors, not cells.

The engineers employed a molecular dynamics method that screened for candidate compounds from methane for self-assembly into membrane-like structures. The most promising compound they found is an acrylonitrile azotosome, which showed good stability, a strong barrier to decomposition, and a flexibility similar to that of phospholipid membranes on Earth. Acrylonitrile – a colorless, poisonous, liquid organic compound used in the manufacture of acrylic fibers, resins and thermoplastics – is present in Titan’s atmosphere.

Excited by the initial proof of concept, Clancy said the next step is to try and demonstrate how these cells would behave in the methane environment – what might be the analogue to reproduction and metabolism in oxygen-free, methane-based cells.

In part, Stevenson said he was inspired by science fiction writer Isaac Asimov, who wrote about the concept of non-water-based life in his 1962 essay, “Not as We Know It.” I think we can conclude as Asimov would – intelligence formed of life “not as we know it” – but with science grounded in material reality, will develop an understanding of science identical in premises as any of our own species’ physical scientists. Leading or trailing one another the results must be the same since material reality remains the same.

Perceptions can vary widely. An intelligent lifeform evolved through differing chemistry wouldn’t be likely to have the same senses or senses arrayed in the same hierarchy. The possibilities are intriguing.

Truly, a worthwhile adventure. I wish them well.